How much humidity does narrow-leaf zinnia need?

Narrow-leaf Zinnia likes Ambient outdoor — a little above a dry room, but nowhere near tropical. In practice most homes are fine for it, and it only struggles when winter heating pushes the air very dry. Watch the leaf tips rather than a hygrometer: crisp brown edges mean the air is too dry, and everything else means you can leave it alone.

Usually nothing — only step in if the air gets very dry. If tips do start browning, group narrow-leaf zinnia with other plants or move it away from radiators and draughts. A pebble tray helps a little; a small humidifier helps a lot. Neither is needed unless the plant is telling you it is dry.

Treating narrow-leaf zinnia as a tropical diva. Over-humidifying a plant in this middle band, especially in a cool room with little airflow, causes more mould and rot problems than dry air ever would.

Crisp brown leaf edges and browning tips are the reliable tell. Yellowing whole leaves is usually a watering issue rather than dry air, and slow growth in winter is normally low light — humidity gets blamed for a lot it does not cause.